Recent growing demand for transmission capacity has led to a demand for faster communication systems. In a network area close to end users, such as an access network, broadband transmission having a communication speed exceeding 100 gigabits per second is expected. However, as the baud rate rises, faster response speeds of optical devices and electronic device used for transmission are demanded. The faster the operation speeds of devices are, the higher the prices of devices are.
To address this issue, modulation methods such as pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) in which the intensity of a signal to be transmitted is varied in order to achieve multilevel intensity modulation have attracted much attention as signal multiplex transmission techniques. Particularly, PAM uses only intensity information, and consequently multilevel intensity modulation is simply realized merely by varying the signal amplitude. By using PAM, a high bit rate may be achieved relatively simply. For this reason, PAM has attracted more attention. Instead of communication in a binary form only using 0 and 1, communication with multilevel modulation by using a modulation method such as PAM enables a decrease in baud rate. As a result, a low-cost, high bit-rate communication system may be implemented using optical devices and electronic devices having low frequency response characteristics.
However, in PAM, since multilevel symbols are assigned to portions where communication has been performed in a binary form using 0 and 1, the distance between symbols is decreased. This results in degradation in signal-to-noise ratio (SNR). Moreover, in order to obtain uniform transmission characteristics for any multilevel symbol, intermediate levels between the level “0” (OFF) and the level “1” (ON) are important. In the case of the level “0”, a signal of the level “0” is generated by turning off the signal source, and thus the level “0” is sometimes represented as “OFF”. In the case of the level “1”, a signal of the level “1” is generated by turning on the signal source, and thus the level “1” is sometimes represented as “ON”. In the case of multilevel intensity-modulated signals as in PAM, optical devices and electronic devices that operate linearly are to be used for not only two levels, 0 and 1, but also the entirety of levels therebetween. In binary transmission methods, only whether the level is 0 or 1 is important, and accuracy in intermediate output levels is not important. If light output does not linearly vary with respect to voltage, this nonlinearity has no influence on the transmission characteristics. However, in the case where optical devices and electronic devices that have been used for communication in a binary form using 0 and 1 are used for multilevel intensity modulation, the intermediate output levels do not vary linearly, which results in degradation in characteristics.
In other words, multilevel intensity modulation has to use the dedicated optical and electrical devices capable of performing linear operations. For this reason, there is a problem in that multilevel intensity modulation is expensive.
To address such a problem, the following related art example is known, for example, Japanese Laid-open Patent Publication No. 10-209961. A signal that is distorted in advance by passing through a nonlinear circuit is inputted to an optical modulator, and thus an increase in each optical power level of a multilevel optical signal transmitted from an optical transmitter is set such that the increase grows monotonously. This alleviates the influence of added noise so as to improve the reception sensitivity.
The following another related art example is known, for example, Japanese Laid-open Patent Publication No. 2008-113386. An optical transmitter includes a data transmission unit that outputs transmission data and a multilevel setting unit that sets the multiple level of an optical signal. The optical transmitter further includes a driving unit that converts an output of the data transmission unit into a multilevel amplitude signal based on an output of the multilevel setting unit, and a light emitting unit that converts an output signal of the driving unit into an optical signal. An optical receiver includes a light reception unit that receives an optical signal from the light emitting unit, a first extinction ratio detector that detects the extinction ratio of an optical signal from an output signal of the light reception unit, and an identification level setting unit that sets an identification level for determining the multilevel of an output signal from the light reception unit based on an output from the extinction ratio detector. The optical receiver further includes a multilevel determination unit that determines the multiple level of an output signal from the light reception unit based on an output of the identification level setting unit. This related art example provides the optical transmitter, the optical receiver, and an optical transmission system in which a multilevel amplitude-modulated optical signal with the SNR being equal among multiple levels is obtained by detecting the extinction ratio of a multilevel optical signal outputted from the optical transmitter and setting a multiple level.